1. Field of the Invention
This invention relates generally to an apparatus and method for measuring optically active materials. More particularly, this invention relates to an apparatus and method for measuring optically active materials, such as dark sugar solutions, that are highly absorbent at visible light wavelengths.
2. Description of the Prior Art
Certain solutions, e.g. sugar solutions, possess the property of being able to rotate the plane of vibration of polarized light. The degree of rotation of the plane of vibration per unit distance of the solution traversed by the polarized light varies in accordance with the strength of the solution. It is well known in the art to use a device having a light polarizer and an analyzer to measure the rotation of the plane of vibration of light passing through a solution, and to thereby determine the percent sugar content of the solution. One example of such an apparatus is shown in U.S. Pat. No. 3,411,342 issued to Theodore Liermann on Nov. 19, 1968. In the Liermann patent, the polarimeter consists of a mercury vapor lamp light source, a collimating lens, a primary polarizer to establish a reference point for measurement of optical rotation, a sample cell through which a continuous stream of crude syrup is circulated, and a measuring circuit that determines the extent of optical rotation caused by the sample and provides an appropriate output signal.
Such conventional polarimeters of the prior art have generally used "visible" light sources in the 500-633 nanometer wavelength emission range. Since dark sugar solutions, such as molasses, are highly absorbent at these wavelengths, it is virtually impossible to transmit a sufficient amount of visible light through such dark sugar solutions in order to measure their optical rotation. The prior art has addressed this problem by requiring clarification of the dark solutions, such as by the addition of lead subacetate Pb(C.sub.2 H.sub.3 O.sub.2).sub.2 .multidot.2Pb(OH).sub.2. However, the use of lead subacetate as a clarifying agent to make the solution transparent at visible light wavelengths presents numerous problems in the health safety area, particularly in waste disposal. Use of such clarifying agents also create the possibility of error in the analysis since the addition of the clarifying agent may affect the true polarization rotation reading. Clarification also adds to the expense of testing the optically active material and increases the length of time required to perform the testing, thereby making such test methods unattractive to continuous flow-through testing operations such as would be present in a factory testing environment.
To date, there has been no known apparatus for measuring the optical rotation of dark sugar solutions, without requiring clarification of the solution before the rotation is measured.
The present invention addresses the problems associated with the prior art devices and methods which required pre-measurement clarification of the dark sugar solutions. The present invention provides a simple, accurate and inexpensive technique and apparatus for measuring the optical rotation of a dark sugar solution without requiring clarification of the sample solution. The present invention thus eliminates the concerns of waste disposal of toxic clarifying agent containing solutions, and is particularly suitable for measurement of continuous flow-through solutions such as found in factory environments.